Research

Automated Video-Based Survey of Travel Times in HOV vs. General Purpose Lanes

Transformation Systems, Inc. was hired to perform automated video-based travel time surveys for the high occupancy vehicle (HOV) and the adjacent general purpose (GP) traffic lane on two freeway corridors in the Seattle area. The purpose of the project was to demonstrate and test the use of machine-vision technology for travel time data collection. Evaluating techniques for determining travel times and speeds is necessary for the WSDOT to determine whether its HOV lane performance criteria are being met. The project used hi-8 video cameras to collect very-high quality video tapes of vehicle license plates. The video tapes were then processed at Computer Recognition Systems, Inc. in Cambridge, Massachusetts, with a specialized computer programmed to convert the video images into computer data files. The report details the video survey methodology; the data acquired through license plate recognition, including date, time, location, and direction of travel; and the comparison data used to determine respective travel times for HOV and GP traffic lanes.

The field data were collected from Monday, June 19, 1995, through Thursday, June 22, 1995, for each four-hour morning peak period (6:00-10:00 AM) on westbound SR 520 and for each four-hour evening peak period (3:00-7:00 PM) on northbound I-5. In all, over 90,000 license plates were analyzed, representing approximately 75 percent of the traffic volume. The license plate data were used to "match" vehicles between camera stations and to compute travel times on more than 200 vehicles per hour in high volume traffic conditions.

Results on northbound 1-5 indicate that the evening peak period commute away from downtown Seattle in the GP lane can be 2.0 to 2.7 times longer than the same commute using the HOV facilities for the 5.75 kilometers (5.75 miles) of freeway monitored (between NE 117th and NE 185th Street). Results averaged over the four weekdays for the entire four-hour evening period indicate approximately 60 percent longer commute times in the GP lanes compared to HOV lanes (the I-5 inside HOV lane has a two-person minimum occupancy requirement).

Similar results on westbound SR 520 could not be statistically verified for the morning peak period because of the relatively low number of vehicles observed during the survey period (transit buses were not counted and the SR 520 outside HOV lane has a three-person minimum occupancy requirement). However, there was evidence of approximately 50 percent longer commute times in the GP traffic lanes between 7:30-8:00 AM than the vehicles observed in the HOV lane on SR 520 for the 1.75 kilometers (1.09 miles) of freeway monitored (between 92nd Ave. NE and 76th Ave. NE).